In the primary loop of pressurized water reactors (PWR), an aqueous solution of boric acid is used to absorb neutrons, which converts their kinetic energy into heat. From time to time this boric acid solution must be removed and disposed of because it becomes contaminated and may corrode the equipment or leave deposits in the tubing. Once the boric acid solution leaves the reactor it is evaporated and vacuum cooled in a crystallization process to reduce the volume of liquid that must be handled. Some of the boric acid precipitates, producing a slurry which may contain up to about 70 wt. % boric acid plus various radioactive and non-radioactive contaminants that may have entered the water.
The Nuclear Regulartory Commission (NRC) requires that low level radioactive waste, such as this boric acid solution, be converted into a free standing monolith for transportation and disposition. It has been the practice in the industry to convert the slurry into a solid monolith by adding cement to it. However, if the boric acid concentration in the slurry is greater than about 30% by weight the cement very rapidly assumes a false set, a thickening and hardening which is not due to the curing of the cement. The false set makes it virtually impossible to move the mixture through pipes, as is normally done in filling containers used for disposal.